Nelson W J, Hammerton R W, McNeill H
Department of Molecular and Cellular Physiology, Stanford University School of Medicine, California 94305-5426.
Soc Gen Physiol Ser. 1991;46:77-87.
Vectorial function of polarized transporting epithelia requires the establishment and maintenance of a nonrandom distribution of Na,K-ATPase on the cell surface. In many epithelia, the Na,K-ATPase is located at the basal-lateral domain of the plasma membrane. The mechanisms involved in the spatial organization of the Na,K-ATPase in these cells are poorly understood. We have been investigating the roles of regulated cell-cell contacts and assembly of the membrane-cytoskeleton in the development of the cell surface polarity of Na,K-ATPase. We have shown that the Na,K-ATPase colocalizes with distinct components of the membrane-cytoskeleton in polarized Madin-Darby canine kidney (MDCK) epithelial cells. Significantly, we showed directly that Na,K-ATPase is a high affinity binding site for the membrane-cytoskeletal proteins ankyrin and fodrin, and that all three proteins exist in a high molecular weight protein complex that also contains the cell adhesion molecule (CAM) uvomorulin. We have proposed that these interactions are important in the assembly at sites of cell-cell contact of the membrane-cytoskeleton, which in turn initiates the development of the nonrandom distribution of the Na,K-ATPase. To directly investigate the functional significance of these protein-protein interactions in the spatial organization of the Na,K-ATPase, we analyzed the distribution of the Na,K-ATPase in fibroblasts transfected with a cDNA encoding the epithelial CAM, uvomorulin. Our results showed that expression of uvomorulin is sufficient to induce a redistribution of Na,K-ATPase from an unrestricted distribution over the entire cell surface in nontransfected cells to a restricted distribution at sites of uvomorulin-mediated cell-cell contacts in the transfected cells; this distribution is similar to that in polarized epithelial cells. This restricted distribution of the Na,K-ATPase occurred in the absence of tight junctions, but coincided with the reorganization of the membrane-cytoskeleton. These results support a model in which the epithelial CAM uvomorulin functions as an inducer of cell surface polarity of Na,K-ATPase through cytoplasmic linkage to the membrane-cytoskeleton.
极化转运上皮细胞的矢量功能需要在细胞表面建立并维持钠钾ATP酶的非随机分布。在许多上皮细胞中，钠钾ATP酶位于质膜的基底外侧区域。这些细胞中钠钾ATP酶空间组织所涉及的机制尚不清楚。我们一直在研究受调控的细胞间接触以及膜细胞骨架组装在钠钾ATP酶细胞表面极性形成中的作用。我们已经表明，在极化的麦迪逊-达比犬肾（MDCK）上皮细胞中，钠钾ATP酶与膜细胞骨架的不同成分共定位。重要的是，我们直接证明钠钾ATP酶是膜细胞骨架蛋白锚蛋白和血影蛋白的高亲和力结合位点，并且这三种蛋白质存在于一个高分子量蛋白质复合物中，该复合物还包含细胞粘附分子（CAM）桥粒芯蛋白。我们提出，这些相互作用在膜细胞骨架的细胞间接触位点的组装中很重要，这反过来又启动了钠钾ATP酶非随机分布的形成。为了直接研究这些蛋白质-蛋白质相互作用在钠钾ATP酶空间组织中的功能意义，我们分析了用编码上皮CAM桥粒芯蛋白的cDNA转染的成纤维细胞中钠钾ATP酶的分布。我们的结果表明，桥粒芯蛋白的表达足以诱导钠钾ATP酶从未转染细胞中在整个细胞表面的无限制分布重新分布到转染细胞中桥粒芯蛋白介导的细胞间接触位点的受限分布；这种分布类似于极化上皮细胞中的分布。钠钾ATP酶的这种受限分布在没有紧密连接的情况下发生，但与膜细胞骨架的重组同时出现。这些结果支持了一个模型，即上皮CAM桥粒芯蛋白通过与膜细胞骨架的细胞质连接作为钠钾ATP酶细胞表面极性的诱导物。
